drm/i915/skl: Implementation of SKL DPLL programming

This patch implements SKL DPLL programming that includes:
        - DPLL allocation
        - wide range PLL calculation and programming
        - DP link rate programming
        - DDI to DPLL mapping

v2: Incorporated following changes
        - Added vfunc for function required outside
        - Fixed multiple comments in WRPLL calculation

v3: - Fix the DCO computation
    - Move the initialization up to not clobber the computed values
    - Use the correct macro for DP link rate programming.
    - Use wait_for() to wait for the PLL locked bit

v4: Rebase on top of nigthly (Damien)

v5: A few code cleanups in the WRPLL computation (Damien)
    - Use uint32_t when possible
    - Use abs_diff() in the WRPLL computation
    - Make the 64bits divisions use div64_u64()
    - Fix typo in dco_central_feq_deviation (freq)
    - Replace the chain of breaks with a goto

v6: Port of the patch to work on top of the shared DPLLs (Damien)
v7: Don't try to handle eDP in ddi_pll_select() (Damien)
v8: Modified as per review comments from Paulo (Satheesh)
v9: Rebase on top of Ander's clock computation staging work for atomic (Damien)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v3)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>